Elemental sulfur occurs naturally in deposits over salt domes, in volcanic deposits, and in evaporate basin deposits associated with calcite, gypsum and anhydrite. The oldest and still most widely used process for the recovery of sulfur from free sulfur containing ore bodies is the Frasch process. In this procedure, which is applied in the recovery of sulfur in situ from subterranean sulfur deposits such as those associated with salt domes in the gulf coast region of the United States, hot water under pressure is injected into the sulfur deposit at a temperature sufficient to melt the sulfur. The pressure maintained on the hot water injection process is sufficient to force the molten sulfur to the surface along with returning Frasch water.
For recovery of sulfur from surface or near surface deposits, or where the Frasch process is considered to be uneconomical because of its energy intensive nature or because the ore matrix is impervious to water, various other recovery procedures have been proposed. For example, sulfur recovery from surface or near surface deposits such as volcanic deposits, has been carried out employing distillation, autoclaving, flotation, agglomeration and solvent extraction. Distillation and autoclaving are, like the Frasch process, energy intensive. Flotation or agglomeration procedures usually yield a product which is not of sufficient purity to be used directly in most chemical process applications.
Many different types of solvent recovery processes have been proposed, both for the recovery of sulfur from crushed comminuted ores and for the recovery of sulfur from ores in situ. For example U.S. Pat. No. 3,535,089 to Lewis et al discloses the extraction of sulfur from a comminuted sulfur ore by means of a hydrocarbon solvent. Examples of suitable solvents disclosed in Lewis include cyclohexane, benzene, toluene and chlorinated hydrocarbons such as trichloroethylene, perchloroethylene, and trichloroethane. In the Lewis process, a solid alkaline material such as lime stone, trona, soda ash, borax, sodium carbonate, ammonium carbonate, ammonium bicarbonate or alkali metal oxide is mixed with the ground sulfur bearing ore. The mixture is then fed to a extraction zone which is operated in a continuous mode by moving the ore material through the zone by means of a helical conveyor.
Another sulfur extraction process using a solvent is based upon the extraction of sulfur by halogenated hydrocarbons as disclosed in U.S. Pat. No. 3,578,418 to Cantrell et al. In Cantrell the extraction solvent is a water immiscible chlorinated hydrocarbon such as trichloroethane, tetrachloroethane, and para- or ortho- dichlorobenzene. The extraction procedure is operated as a continuous type process in which hot loaded solvent is withdrawn from the extraction zone and then cooled to precipitate the sulfur in solid form. The sulfur is then washed with a wash liquid such as methanol, acetone or ethylene glycol.
Yet a further process which represents an improvement upon the process disclosed in the aformentioned patent to Lewis et al is disclosed in U.S. Pat. No. 3,619,147 to Amano et al. Here the solvent extraction and sulfur recovery steps are carried out in a closed system in which solvent is recycled without any loss through evaporation. The solvent extraction zone in Amano et al comprises a plurality of serially connected tanks which are operated to provide a multi-stage, semi-batch type operation.
Chemical extraction may be employed in the in situ recovery of sulfur from subterranean deposits of the type which are usually the subject of Frasch processing. For example U.S. Pat. No. 3,645,551 to Thompson discloses the recovery of sulfur employing an aromatic hydrocarbon which is injected into the deposit at an elevated temperature at which it reacts with sulfur to form hydrogen sulfide. The in situ reaction is carried out in the presence of sufficient water so that carbon dioxide is formed along with the hydrogen sulfide.